Carbon fibers have excellent properties including high crystallinity, high electrical conductivity, high strength, high modulus, light weight, etc. In particular, ultrafine carbon fibers (carbon nanofibers) are used as nanofillers for high-performance composite materials. The application thereof is not limited to the conventional use as reinforcing nanofillers for improving mechanical strength. Taking advantage of the high electrical conductivity of a carbon material, they are expected to be applied as electrically conductive resin nanofillers for electrode additive materials for batteries, electrode additive materials for capacitors, electromagnetic shielding materials, and antistatic materials, or as nanofillers for electrostatic coating for resins. Further, taking advantage of the characteristic chemical stability and thermal stability with the fine structure as a carbon material, they are also expected to be used as field electron emission materials for flat displays, etc.
As methods for producing such ultrafine carbon fibers as a high-performance composite material, the following two methods have been reported: 1) a method for the production of carbon fibers using a vapor-phase process (Vapor Grown carbon Fiber; hereinafter referred to as VGCF); and 2) a method for production by melt-spinning a resin composition (mixture).
As production methods using a vapor-phase process, the following methods have been disclosed, for example: a method in which using benzene or a like organic compound as a raw material, ferrocene or a like organotransition metal compound is introduced as a catalyst into a high-temperature reactor with a carrier gas, thereby producing carbon fibers on the base (see, e.g., Patent Document 1); a method in which VGCF is produced in a floating state (see, e.g., Patent Document 2); a method in which carbon fibers are grown on the wall of a reactor (see, e.g., Patent Document 3); etc. Although ultrafine carbon fibers obtained by these methods have high strength and high modulus, there is a problem that each fiber has a number of branches, resulting in poor performance as reinforcing fillers. There also is the problem of high cost due to productivity. Further, production methods using a vapor-phase process are problematic in that purification is required in some fields of application because of the presence of a metal catalyst or carbonaceous impurities in VGCF, and such purification increases the cost.
Meanwhile, as a method for producing carbon fibers by melt-spinning a resin composition (mixture), a method in which ultrafine carbon fibers are produced from composite fibers of phenol resin and polyethylene has been disclosed (see, e.g., Patent Document 4). The method provides ultrafine carbon fibers with a less branched structure. However, there are problems that, for example, because phenol resin is completely amorphous, orientation formation is difficult, and also, because it is a non-graphitizing carbon, strength and modulus cannot be expected from the resulting ultrafine carbon fibers. In addition, there also is a problem that because phenol resin is insolubilized (stabilized) via polyethylene in an acidic solution, the diffusion of the acidic solution in polyethylene is rate-limiting, and insolubilization takes a long period of time, etc.
(Patent Document 1) JP-A-60-27700 (official gazette, pp. 2-3)
(Patent Document 2) JP-A-60-54998 (official gazette, pp. 1-2)
(Patent Document 3) Japanese Patent No. 2778434 (official gazette, pp. 1-2)
(Patent Document 4) JP-A-2001-73226 (official gazette, pp. 3-4)